The invention relates to an induction lamp system comprising a power supply unit, which can supply a high-frequency alternating voltage, a power supply cable connected to the power supply unit, and an induction lamp connected to the power supply cable and comprising an induction coil, at least one electric component being arranged between the power supply cable and the induction coil.
Induction lamps are known by virtue of their long lifetime and high efficiency which is comparable with conventional gas discharge lamps such as TL tubes. The operation of an induction lamp is also comparable therewith to a certain extent, be it that the electron current in the lamp is not generated by a voltage difference between two electrodes but by an induction coil in which a high frequency voltage is generated causing a current to flow in the gas which is enclosed in the glass envelope around the coil. The gas may be mercury vapor (coming from an amalgam) and (rare) gas. The emission of electromagnetic radiation by the gas as a result of the electric current and the conversion of this radiation into visible light by means of fluorescence is entirely comparable with a conventional gas discharge lamp.
The lamp impedance is dependent, inter alia, on the rare gas mixture and the mercury vapor pressure. The latter varies with the temperature of the amalgam. Heating of the lamp, after it has been switched on, thus initially causes fluctuations of the electrical behavior, which eventually stabilizes. The electric power supply, usually incorporating a high frequency voltage generator and a resonant circuit, is to be designed in such a way that the lamp is fed with the correct power at different impedances.
An induction lamp system as described in the opening paragraph is known from international patent specification WO 93/23975. In this system, a capacitor is arranged in series with the induction lamp and the capacitor has such a value that the lamp having a predetermined fixed value (for example, 85 W) resonates at the same frequency as the power supply unit. A property of such assemblies is that the length of the power supply cable influences the impedance which is xe2x80x9cseenxe2x80x9d by the power supply unit, and hence influences the resonance frequency. The value of the capacitor and of the components in the power supply unit must therefore be adapted in advance to the cable length to be used. Consequently, the cable length is predetermined for a given system, which is detrimental to the flexibility of consumer uses of such a system.
It is an object of the invention to provide an induction lamp system for a high frequency induction lamp which is suitable for different cable lengths. It is another object of the invention to provide an induction lamp system which is independent of cable lengths and is inexpensive and has a high efficiency.
To this end, the electronic component has such properties that the absolute value of the imaginary component Im(Z) of the overall impedance Z of the power supply cable, the electronic component and the induction coil is smaller than the real component Re(Z) for any arbitrary length of the power supply cable. The absolute value of the imaginary component Im(Z) of the overall impedance Z is preferably smaller than 0.50 times the real component Re(Z), more preferably smaller than 0.25 times the real component Re(Z). The aim is a completely pure real impedance so that the resonance properties of the system are entirely independent of the length of the power supply cable if use is made of a coax cable. Since the imaginary impedance is also dependent on the temperature and the mercury vapor pressure in the lamp, which increases during start-up of the lamp, this is not feasible for any arbitrary temperature, but by correct choice of the components, the imaginary component Im(Z) can be maintained within acceptable limits for any occurring temperature. The wording xe2x80x9carbitrary length of the power supply cablexe2x80x9d is to be understood to mean at least any cable length between 0.2 and 4.0 meters.
The value of the real component Re(Z) of the overall impedance Z of the electronic component and the induction lamp preferably differs less than 15%, more preferably less than 8% from the characteristic impedance of the power supply cable. The two measures mentioned above, namely the substantial elimination of the imaginary component of the impedance and rendering the real component of the impedance of the induction lamp substantially equal to the characteristic impedance of the cable is referred to as characteristic termination, said measures jointly ensuring that the length of the power supply cable does not influence the impedance of the system.
The variation of the absolute value |Z| of the overall impedance Z is preferably smaller than 35xcexa9 for different cable lengths. This is advantageous because the light output of the lamp is also influenced by |Z| and an approximately equal light output is generally desired for any cable length.
In a preferred embodiment, the electronic component comprises a capacitor. The above-mentioned aims can be realized in an efficient manner, inter alia, by interpositioning a capacitor of the correct value.
The power supply unit preferably comprises a resonant circuit which is adapted to the overall impedance Z for an optimal light output, the resonant circuit preferably comprising two coils and one capacitor. It has been found that this is the minimum number of components to achieve the desired result with a sufficient efficiency. Overall, the system will thus comprise two coils, two capacitors and one induction lamp (which may also be considered to be a coil).
The invention also relates to an induction lamp comprising an induction coil and an electronic component, which induction lamp can be connected by means of a power supply cable to a power supply unit, which can supply a high frequency alternating voltage, the electronic component being arranged between the induction coil and a connection point for the power supply cable, said electronic component having such properties that the absolute value of the imaginary component Im(Z) of the overall impedance Z of the power supply cable to be connected, the electronic component and the induction coil is smaller than the real component Re(Z) for any arbitrary length of the power supply cable.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.